1. Field of the Invention
The present invention relates to a control apparatus for a vibration wave actuator.
2. Related Background Art
In general, a vibration wave (or type) actuator includes a vibration member in which drive vibrations are produced and a contact member pressed against the vibration member, and is designed to move the vibration member and contact member relative to each other by using the drive vibrations.
The vibration member is generally comprised of an elastic member and piezoelectric elements serving as electromechanical energy conversion elements. For example, piezoelectric elements having driving phases at positions with a spatial phase difference of 90xc2x0 are arranged on the elastic member, and two-phase alternating signals having a phase difference of 90xc2x0 are applied to the two driving phases, thereby producing travelling waves on the elastic member. The contact member is then pressed against the elastic member to obtain a driving force by a frictional force.
Note that a friction material for obtaining a proper frictional force is bonded, applied, or formed on the contact portion between the vibration member and the contact member.
According to a conventional method of detecting the vibration amplitude of the vibration wave actuator, an electrode for vibration detection is provided for a piezoelectric element, and the value obtained by converting mechanical energy accompanying vibrations into electrical energy is used as vibration detection information. For example, vibration detection information is compared with a threshold voltage by a comparator to detect whether the voltage represented by the information is higher or lower than the threshold voltage, as disclosed in Japanese Patent Application Laid-Open No. 1-303074, or the information is rectified and converted into a DC voltage, and the voltage is A/D-converted to be detected, as disclosed in Japanese Patent Application Laid-Open No. 3-22870.
In the method of detecting vibration amplitude disclosed in Japanese Patent Application Laid-Open No. 1-303074, since the result obtained by comparator only makes it possible to determine whether the information is higher or lower than a predetermined level, in order to accurately know vibration amplitude, for example, a plurality of thresholds must be prepared to detect amplitude. This requires many comparators.
In the method of detecting vibration amplitude disclosed in Japanese Patent Application Laid-Open No. 3-22870, a driving waveform signal is rectified and smoothed first, and then the resultant signal is read into a computer through an A/D converter. This requires a complicated, expensive circuit. In addition, since the signal is smoothed, detection generates a delay.
According to one aspect of the invention, there is provided a control apparatus for a vibration wave actuator which excites a vibration member and obtains a driving force by applying a frequency signal to a driving electromechanical energy conversion element mounted on the vibration member, comprising a driven state detecting unit including a monitor unit which outputs a waveform signal having an amplitude corresponding to the vibration state of the vibration member, a pulse width forming circuit which forms a pulse width signal by comparing a signal from the monitor unit with a threshold, and a determining circuit which determines the driven state in accordance with the pulse width, wherein a driven state (e.g., the amplitude of a waveform signal corresponding to the vibration state) is detected based on the pulse width.
The above and other objects, features, and advantages of the present invention will be apparent from the following detailed description in conjunction with the accompanying drawings and the appended claims.